Motors, generators and the like typically comprise a rotor rotatably mounted within a stator core. The stator core comprises stator windinqs consisting of insulated bundles of high voltage conductors, known as stator bars, embedded in slots in the stator core.
The insulation surrounding such high voltage conductors is subject to deterioration over time. Deterioration of the insulation surrounding a conductor can result in partial discharge activity within the insulation, which further deteriorates the insulation. Thus, a vicious circle is established, whereby as the insulation deteriorates partial discharge activity increases in severity and thus increases the rate of deterioration of the insulation. Eventually the insulated conductor experiencing such partial discharge activity must be replaced to avoid or correct a fault in the stator windings.
Replacement of a stator bar is a costly and time consuming process, requiring that the motor or generator be taken out of service and dismantled. Once a fault occurs in the stator windings, replacement must be effected in order to restore the motor or generator to operating condition. It is thus advantageous to be able to determine the condition of the insulation in the stator windings in advance in order to anticipate whether and approximately when repair will be necessary, so that repair can be effected in an orderly and organized fashion before the fault becomes catastrophic and at a time which is most convenient having regard to the operating schedule of the particular motor or generator concerned.
The severity of partial discharge activity in the stator windings is generally accepted as a good indicator of the condition of the high voltage insulation surrounding conductors in the windings. Previously, high voltage coupling capacitors have been employed at the point of energization of the stator windings to block the high 60 Hertz voltage while permitting any high frequency partial discharge signal to be coupled to a measuring instrument. Such capacitors are bulky and expensive, and require direct connection to the high voltage conductors of the windings, thus reducing the integrity of the motor or generator insulation system.
Moreover, such capacitors cannot distinguish between the high frequency partial discharge signals and background interference. Furthermore, where partial discharge activity is detected in this manner, its location and severity can only be determined by physical inspection, requiring that the motor or generator be taken out of service and dismantled.
The present invention overcomes these disadvantages by providing a partial discharge detection method and device adapted to detect partial discharge within a particular stator slot or adjacent stator slots from the region of the stator slot itself. The present invention thus not only detects partial discharge activity within a stator slot, but provides a good indication of the particular stator slot in which such partial discharge activity is occurring. The device is inherently insensitive to electrical interference which originates outside the stator slot and which can produce a misleading indication of the condition of the high voltage insulation.
This is accomplished in the present invention through a detection device comprising a signal conductor spanning a conductive ground plane adapted to transmit to a recording instrument such as an oscilloscope a signal electromagnetically coupled from high frequency components of a pulse generated by partial discharge activity within a stator slot. The detection device is installed in or adjacent to the stator slot, which effectively restricts the measurement of partial discharge activity to electrical activity occurring within that particular stator slot or a nearby stator slot.
Since high frequency signals are severely attenuated as they propagate within a stator slot, the signal conductor, designed to be sensitive only to high frequency signals, is sensitive only to partial discharge activity within the stator slot or a nearby stator slot, and is relatively insensitive to partial discharge activity originating elsewhere or background interference. The partial discharge detection device and method of the present invention can thus distinguish between partial discharge signals and background interference. Moreover, since there is no direct connection to any high voltage conductor in the stator windings, there is no effect upon the integrity of the motor or generator insulation system. The partial discharge detection device of the present invention is also relatively small, inexpensive and relatively simple to install.
A partial discharge detection device embodying the present invention may be implemented into the construction of a stator, or may be retrofitted to existing stator cores which have heretofore relied upon high voltage coupling capacitors to detect partial discharge activity. A preferred embodiment of the invention provides a relatively slender conductive ground plane and signal conductor, which will easily fit within the clearance between the rotor and the stator core of an existing motor or generator without adversely affecting the operation thereof.